The present invention relates to a power supply device. Specifically, the power supply device performs PFM (pulse frequency modulation) control relative to a switching element by using a digital circuit.
A known power supply device is disclosed in Japanese Patent Publication Number 2006-295802. This power supply device has a pulse control device that has a clock generator, a pulse driving signal generation unit and a PFM (pulse frequency modulation) control unit. In this pulse control device, the clock generator generates a clock signal, which has any frequency, based on a pulse that is output from an oscillator (not shown) such as a crystal oscillator and supplies the clock signal to the pulse driving signal generation unit. The pulse driving signal generation unit generates a pulse driving signal based on the clock signal that is input and outputs the pulse driving signal to a switching element. Specifically, the pulse driving signal has a switching frequency that is set by the PFM control unit. The PFM control unit performs a PFM operation (calculation), which corresponds to a digital arithmetic process, by sampling a feedback signal that is induced at a secondary winding in a feedback transformer every control cycle. Then, the PFM control unit sets the calculated switching frequency in the pulse driving signal generation unit.
In this pulse control device, first of all, the PFM control unit performs sampling by A/D converting the feedback signal. Next, based on information of an output power after performing sampling, the PFM control unit performs the PFM operation for a pulse frequency (a switching frequency) of the pulse driving signal, which corresponds to a PFM pulse, determines a PFM pulse frequency and sets it in the pulse driving signal generation unit. The operations of the PFM control unit with respect to the determination and setting explained above are repeated. In this case, specifically, the pulse driving signal generation unit counts the clock signal that is input, and at the same time, generates the pulse driving signal by suitably raising or lowering an output signal level according to a count number. Further, as an example, the pulse driving signal generation unit performs a frequency modulation of the pulse driving signal by making a width of an ON period of the pulse driving signal constant and changing a width of an OFF period corresponding to the PFM pulse frequency that is set by the PFM control unit. Therefore, according to the above count number that is counted by the pulse driving signal generation unit, a frequency of the pulse driving signal is defined. Thus, even though Japanese Patent Publication Number 2006-295802 does not disclose it, the above count number that is counted by the pulse driving signal generation unit is considered to be set as a number that shows a determined PFM pulse frequency by the PFM control unit.
The power supply device described above, however, still has some problems to be solved as discussed below. That is, it is necessary in this power supply device that a frequency of the clock signal that is output from the clock generator to the pulse driving signal generation unit must be raised to make an adjustment unit finer in regards to a frequency of the pulse driving signal. However, when the frequency of the clock signal is raised, as a result, an electric part capable of high speed operations needs to be used as an electric part such as a CPU that configures the pulse driving signal generation unit. Therefore, the problems to be solved, such as an increase of noise and an increase of electric power consumption, exist.
The present invention attempts to solve these problems. An object of the present invention is to provide a power supply device that can avoid using high-speed electric parts, and at the same time, that can make an adjustment unit finer in regards to a frequency of a driving signal for a switching element.